The purpose of the present invention is to provide an endogenous humanin receptor agonist which shows a stronger activity of inhibiting dysfunction or cell death of a nerve cell than humanin. The present invention relates to a medicinal composition which comprises, as the active ingredient, a peptide having an amino acid sequence represented by formula (I) [formula (I) being given in standard one letter amino acid code]: TGKNLSEAGLRKLI(S or G)EVDSDGD (I) and having an activity of inhibiting dysfunction or cell death of a nerve cell, a polypeptide containing the amino acid sequence represented by formula (I) and having an activity of inhibiting dysfunction or cell death of a nerve cell, or the like.